Polymeric adhesives and paper coatings are used in many paper, paperboard and disposable packaging applications. Numerous adhesives, paper coating binders and glossy coatings or varnishes are used in coated paper, paperboard, the packaging of products such as salt, sugar, tea, coffee and bottle labels, etc. All of these products, and numerous other packaging materials end up for the most part in recycled paper, board and packaging or in municipal solid waste (MSW) streams in landfills. Paper and paperboard represent a significant component (about 35% by volume) of the MSW stream and efforts are underway to recycle certain streams and compost others. These largely cellulosic packaging materials should ideally be designed to be fully compatible with composting or paper recycling operations.
With the rising cost of virgin fiber and the increased demand for wastepaper, the pressure is on to re-use more and more contaminated wastepaper. As a result, contaminant removal, which is essential to convert wastepaper into a reusable fiber, is one of the most important factors influencing the economics of the recycling operation, since this has a direct bearing on the yield of reusable fiber from wastepaper and its total cost. Old newsprint (ONP) is the most abundant used paper fiber source, and is most commonly used for the production of recycled paper. Efficient removal of the ink from ONP can be generally accomplished only by incorporating about 25 to 40% of old magazine (OMG). The OMG contains clays and mineral particles that facilitate the removal of the ink by a flotation de-inking process. The introduction of OMG also improves fiber strength and brightness levels of the recycled fiber. On the other hand, the incorporation of OMG in the recycling process introduces polymer residues from the adhesives and coatings used to manufacture the magazines.
To benefit the environment, adhesives, paper coating binders, varnishes and other polymeric resins used in paper and paperboard applications should be repulpable and not interfere with the recycling process. In addition, they should be biodegradable and have the required cost and performance characteristics to compete effectively in the market place.
Various natural adhesives (starches, dextrins, etc.) and derivatives of natural products which are biodegradable and have adhesive properties, such as carboxymethyl cellulose, amylose from starch, and casein from milk find uses in adhesive applications. Natural adhesives are used in packaging applications, but they continue to be displaced by synthetics primarily due to performance. The same is true for co-binders used in paper coatings, including thermally modified, acid thinned, phosphorylated and ethylated starches. Although they are biodegradable and compostable, these natural adhesives and paper coating binders can cause a problem in paper recycling because they are water soluble, and thus are concentrated in the closed-system water loop of the repulping process where they can build up in the initial section of the dryer and on the dryer felts. This problem is even more severe for synthetic (petroleum based) latex products used in paper since these soft polymers typically elongate and extrude through the basket screens of paper recycling operations [see Bloembergen, S., Nemeth, S. B., and McLennan, I. J., “Second-Generation Repulpable PSAs for Benign USPS Postage Stamps”, Adhesives & Sealants Industry, p. 42-48, (May 2002).]. And this is a problem not only for adhesives, but also for paper coating binders, overprint varnishes, thickeners, rheology modifiers and other synthetic latex additives.
With the growing trend of mills re-using their process water, It is becoming as important to effectively remove all contaminants from the pulp flow as it is to remove them totally from the water system in an effort to prevent the accumulation of colloidal impurities. The preferred approach to achieve this requirement is to separate the contaminants at the earliest possible step in the process, but the inherent sticky nature of currently used synthetic (petroleum based) hot melts, pressure-sensitive adhesive products, and latex binders and emulsions used in paper makes this very difficult. The reduction of water consumption (zero-discharge) with closed water recirculation systems causes reagglomeration of dispersed adhesives and latex binders, resulting in white pitch problems and deposits known as “stickies” on dryer walls and on the polyester ‘wire’, i.e. the felt on which the recycled paper is deposited. This occurs at very high speeds, and once stickies begin to deposit, build-up occurs exponentially leading to costly mill shut downs.
The residues from adhesives, coating binders and other polymeric materials currently used in glossy paper coatings, sizing agents, toner particles, etc., which lead to the formation of “stickies”, can have a major impact on the smooth operation and the economics of a paper recycling process. Currently, centrifugal cleaning and fine screening are regarded as the best systems for stickies removal, but these are costly and inefficient.
U.S. Pat. No. 5,872,199 to Bloembergen et al., U.S. Pat. No. 6,242,593 to Bloembergen et al., and U.S. Pat. No. 6,355,734 to Cassar et al., are all directed to repulpable & biodegradable adhesives and ink resins. In addition to adhesives and ink resins, there is a need for the design of repulpable & biodegradable paper coating binders.
Paper coatings are pigment-containing coatings compositions that are applied onto paper and paperboard to improve their aesthetic appearance and printability. The pigment coatings impart smoothness, gloss, brightness, and opacity to the base sheets for improved appearance, and provide them with enhanced printability which requires resistance to ink film-splitting forces, fountain-solution receptivity, balanced ink setting and holdout, ink gloss, sharp halftone reproduction, etc. For the pigment coatings, pigments and pigment binders are the most important ingredients so that their selections are critical. Pigment binders not only perform the basic required role of binding pigment particles to each other and bonding them to the base sheets, but also significantly influence the rheology, coater runnability, and drying behaviors of pigment coating formulations and the optical, viscoelastic, and printing properties of coated paper and paperboard products. Various types of soft latexes, such as styrene-butadiene (S/B), styrene-butyl acrylate (S/BA), and polyvinyl acetate (PVAc) latexes, are widely used as binders for paper coatings. These three major types of paper coating latexes are often functionalized with monomers containing carboxylic acids (—COOH), amides (—CONH2), hydroxyl groups (—OH), etc. and modified with monomers such as acrylonitrile (VCN), methyl methacrylate (MMA), etc. [see D. I. Lee, “Coating Binders-Latex,” Chapter 19 in “Pigment Coating and Surface Sizing of Paper” edited by Esa Lehtinen for “The Papermaking Science and Technology Book Series,” The Finnish Paper Engineers'Association and TAPPI PRESS, 2000]. S/B latexes are latexes of modified copolymers of styrene (hard monomer) and butadiene (soft monomer) at varying ratios ranging from 40/60 to 80/20. Their glass transition temperatures (Tg's) range from −25 to 50° C. S/A latexes are latexes of modified styrene (hard monomer) and n-butyl acrylate (soft monomer) at varying ratios ranging from 40/60 to 60/40. Their Tg's range from −10 to 40° C. PVAc latexes are mostly homopolymer latexes. Polyvinyl acetate homopolymer Tg's are about 30° C., but their wet latex Tg's (9-11) are about 13° C. so that they are room temperature film-forming latexes despite their high polymer Tg's.
In each type of paper coating latexes, many variations in composition, functional modification, molecular structure, particle size, etc. can be found. For example, there are commercial S/B and S/A latexes having low, medium, and high levels of carboxylation for unique paper coating properties such as high binding strength, high mechanical stability, etc. In order to incorporate polar moieties into and increase the surface energetics of S/B and S/A latex copolymers, they are often copolymerized with either acrylonitrile (VCN) or acrylic acid (AA) or methacrylic acid (MA), along with various other functional monomers. Many carboxylated S/B/MMA/VCN latexes are widely used as paper coating binders in Japan. Among these three types of paper coating latexes, S/B and S/A latexes are very similar in performance except that they have their respective unique properties, but they are quite different from PVAc latexes. [see D. I. Lee, “Coating Binders-Latex,” Chapter 19 in “Pigment Coating and Surface Sizing of Paper” edited by Esa Lehtinen for “The Papermaking Science and Technology Book Series,” The Finnish Paper Engineers' Association and TAPPI PRESS, 2000]. As mentioned, most of PVAc latexes are homopolymer latexes, but they are also available as vinyl acrylic latexes which are vinyl acetate copolymers with ethyl acrylate or n-butyl acrylate and as vinyl acetate ethylene copolymer latexes. They are sometimes lightly carboxylated. Polyvinyl acetate homopolymer and copolymer latexes are not only highly polar and hydrophilic, but also tend to hydrolyze and produce polyvinyl alcohols, especially on the particle surface, and become more hydrophilic. For these reasons, they are highly water-swollen and their particle surfaces are modified with polyvinyl alcohols. These unique properties impart higher viscosity to paper coating formulations and higher porosity to coated papers than their counterpart SIB and S/A latexes. On the other hand, because of the high low-shear viscosity and high-shear dilatant behavior of paper coating formulations, they are limited to lower coating solids than their counterparts.
In addition to their different chemistries, the molecular architecture of S/B, S/A, and PVAc latex polymers is also different [see U.S. Pat. No. 4,478,974. Oct. 23, 1984 to D. I. Lee et al., and U.S. Pat. No. 4,134,872. Jan. 16, 1979 to D. I. Lee]. S/B latex copolymers are crosslinked because of butadiene having two double bonds, whereas S/A and PVAc latex polymers are linear, unless intentionally crosslinked. Because S/B latex copolymers are crosslinked, they can only be characterized in terms of % gel and swell index, along with some information on the molecular weights of their soluble portions. For this reason, S/B latexes for paper coating applications should not be called either S/B rubber latexes or S/B latex rubbers which contain high butadiene (>65%) and are nearly non-crosslinked. Although S/A and PVAc latex polymers are mostly soluble in appropriate solvents and can be characterized by their molecular weights, they are sometimes insoluble because they are intentionally crosslinked. In this case, their gels will be isolated and characterized in terms of swelling index, while their solubles can be analyzed for the molecular weights. These differences in the molecular architecture along with their different chemistries result in differences observed in their paper coating performance among three major types of synthetic paper coating latexes.
S/B latexes are more widely used for paper coatings as binders throughout the world than the other two types of paper coating latexes, S/A and PVAc latexes, but S/A latexes are used more in Europe than in North America and Asia, while PVAc latexes are used more in North America.
The dominant commercially available paper coating binders which are petroleum based latex emulsions still cause stickies problems in closed loop recycling mills. Therefore, there is still a need for repulpable latex binders and coatings that match the performance and cost of the predominantly synthetic products now being used. A truly ‘repulpable’ polymer is a polymer which does not persist as “stickies” in a paper recycling process, but which can be quantitatively removed from the process using conventional equipment found in a paper recycling mill.
In addition, products that are made from ingredients derived from annually renewable crop sources, offer the intrinsic value proposition of a reduced carbon footprint by way of renewable carbon in the product that is in harmony with the rates and time scales of the natural biological carbon cycle.
The present inventors have prepared multiple disclosures regarding the composition and use of various forms of sugar based latex copolymers. For instance, U.S. Pat. No. 5,872,199 describes novel copolymers which are useful in biodegradable, repulpable adhesives, coatings, sizing agents, toners, retention aids and related products used in paper and paperboard applications, in wood gluing and other packaging applications. The copolymers of the '199 patent are represented by the formula

wherein Glu is a saccharide moiety which is derived from α,-D-glucose (dextrose), fructose, mannose, galactose, talose, gulose, allose, altrose, idose, arabinose, xylose, lyxose, ribose, or mixtures thereof, or which can be derived by hydrolysis from the group consisting of starch, corn syrups- or maltodextrins, maltose, sucrose, lactose, maltotriose, xylobiose, mellibiose, cellobiose, raffinose, stachiose, levoglucosan, and 1,6-anhydroglucofuranose. R1 and R2 are substituent groups of a vinyl monomer or mixture of vinyl monomers, wherein said vinyl monomer or mixture of vinyl monomers is selected from the group consisting of vinyl acetate, ethyl hexyl acrylate, butyl acrylate, ethyl acrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, lauryl acrylate, methyl methacrylate, methacryclic acid, acrylic acid, and other acrylates or mixtures of different acrylate monomers, ethylene, 1,3-butadiene, styrene, vinyl chloride, vinylpyrrolidinone, and other vinyl monomers, or mixtures thereof, R is selected from the group consisting of a C1 to C30 alkyl or a mixture thereof, more preferably a C3 to C8 alkyl or a mixture thereof, R′″ is selected from the group consisting of a C1 to C30 alkyl or a mixture thereof, or a hydrogen, preferably a C8 to C18 alkyl or a mixture thereof, and most preferably a C12 to C14 alkyl or a mixture thereof; n is an integer ranging from 0 to 10, its average value ranging from 0.3 to 1; thus, <n+1>=1.3 to 2 corresponds to the average degree of oligomerization of the alkyl polyglycoside; x and y are integers ranging from 0 to 3 or from 0 to 4, where the maximum value of 3 or 4 for x and y equals the number of hydroxyls on the Glu moiety, but not both x and y are zero, and, p and q are integers ranging from 0 to 1000, but not both p and q are zero. The swirly lines indicate continuing polymer chains.
The invention in U.S. Pat. No. 6,242,593 relates to environmentally friendly sugar-based vinyl monomers useful in repulpable adhesives and other applications. However, this invention does not anticipate, consider nor provide any motivation to utilize the sugar macromers for sugar-acrylic latexes as particularly useful paper coating binders, as in the current invention, nor does it anticipate the advantages that result therefrom. Similarly, U.S. Pat. No. 6,355,734 relates to resin-fortified sugar-based vinyl emulsion copolymers and methods of preparing the same for use as inks. This invention also does not anticipate, consider nor provide any motivation to utilize the sugar macromers for sugar-acrylic latexes as particularly useful paper coating binders, as in the current invention, nor does it anticipate the advantages that result therefrom.
Accordingly, a need exists for methods of treating said sugar macromers to provide sugar-acrylic latexes particularly useful as paper coating binders which accommodate this challenge.